Method of forming integrally stiffened panels



M. S. REED April 17, 1956 METHOD OF FORMING INTEGRALLY STIFFENED PANELS Filed June 26. 1952 2 Sheets-Sheet 1 INVENTOR MaedozzaldSReed fiaw iwmw ATTORNEYS M. s. REED 2,741,834-

METHOD OF FORMING INTEGRALLY STIFFENED PANELS 2 Sheets-Sheet 2 April 11, 1956 Filed June 26, 1952 INVENTUR MaalazzaldS.l{eed BY ATTORNEYS United States Patent 0 2,74rss4 METHOD OF FGRB'ENGINTEGRAELLX S- EIEFENED PANEIS Mhcdonaid S. Reed-,Ei'ie, Be.

Application June 26, 1952, SeriahNo. 2%,699

1 Claims. (ill. 29-548)" This inverrnon relates to methods of forging sheets and particularly to the forging of thin' sheets having integral upstanding ribs, usually referredto as integrallyotilfened panels;

It is desirable, especially in the aircraft industry, to have sheets ofv relatively large surface area and quite thin provided with integral upstanding ribs or flanges formed integrally therewith to provide stiffening whereby' such sheets may be employed as a rigid skin for the aircraft. The forging of such structural sheets has heretofore been impractical by known methods since the finished sheet requirements as relating to the thickness of the integral flanges and the heights thereof is such as to' render such shapes impractical to forge by anypreviously known methods; In general, itis" desirable that finished sheets h of a thickness of the order of magnitude of ):;2 inch be provided with integral ribs or flanges of about the same thickness projecting from one surface and spaced apait from 1'1": to 3' inches. Theribs are desirably of a height ofth'e general order. of '1 inch or more. It can readily be seen that integral structural shapes of those dimensions are extremely dirl'icult to forge by practical methods;

F orging directly to final form hash'eretofore been considered as a possible means of producing integrallystilfened panels or" the required type: but the previous techniques limite'dlthe thickness of the sheets to a value 40 considerably greater than that desired and definite limitations existed as to the possible spacing between parallel ribs. Further limitations were encountered limiting the ratio of the height of the rib or flange to its thickness to values below those desired. All such operations involving forging of sheets to the required dimensions. resulted in defects known as. suck-in at the base of the ribs or flanges and impractically high pressures were required. In fact, the unit pressures required were so high 50 that there was an appreciable compression and. deflection of the die, even with the best die steels available, and as s result of this die deflection. sheets of uniform thickness could not be forged, the sheets being thicker inthe center ofithe area. As a result of. this when it is attempted. to forge an'integrally stiffened sheet from a sheet ofuniform thickness, as is desirable, if the original provides the proper volume of metal near the edge of the die, the volume: of metal available to make the ribs near the center of the die is not sufficient, due to the greater thickness and volume of the skin at that point. In causing" the material to flow until the skin thickness reached the desired values, the surface of the sheet opposite each rib developed depressions where the surface material was drawn into the cavity, thus resulting in a product having grooves'on the face opposite the base of each rib. Such defects" render the product unacceptable to the aircraft industry.

The present invention relates to a manner of forging such shapesby amethod resulting in acceptable products of the desired dimensions without the defects of suckin. ft isknown thatthe pressure required to cause flow Patented Apr. 17, 1956 of material being'processed by press forging is inversely proportional to the ratio of the least lateral dimension of the material to the. material thickness in the direction of die travel-. Iirthe present case, the least lateral dimension of the material can be taken as the distance between adjacent ribs. ness in sheet material of inch thickness requires considerably less total pressure than that necessary to effect a reduction in thickness in a sheet of equal area but of inch thickness.

The present invention contemplates preforming' a sheet of stock. material, such as by forging, rolling or in some. other suitable manner to provide enlargements or projections on one or both faces thereof comprising concentrations of material suflicient to form the stiffening ribs of the finished product. Such preformed sheets are placed between the dies of a forging press or hammer wi'ththe enlargementslocated at the entrance openings of grooves provided in the dies for forming the final ribs. The dies. employed are further provided with ribs and grooves-between the aforementioned rib grooves to corrugat'e the sheet between the rib positions while at the same time forging those portions to the required thin section. The corrugating of the sheet between the ribs is performed by'stretching and bending of the material while it is being forged. After the sheet is stretched and forged, it may be removed from the dies and stretchedor treated to flatten. thecorrugations between the ribs and thus provide the required structural shape consisting of a.fiat. thin sheet having integral upstanding ribs projecting from one face thereof.

By corrugating the sheet portions between the ribs'during the forging operation, the finished product is of greater dimension inone direction than the dies employed and may even exceed the entire bed area of the press. For example, if in the final stretching operation the width of. the sheet is doubled, then due to thatfactor alone, a press of only half the tonnage capacity could be used to make the same sheets. Furthermore, by concentrating the metal at the rib positions minimum flow of the metal is. necessary during the forging step. An additional feature of the present method lies in the fact that forging of the sheet to reduced thickness between the ribs is accomplished by applying pressure in such direction to the surface of the sheet that the thickness of the metal. between the die, measured in the direction of relative die travel, is considerably greater than the actual thickness of the corrugated sheet measured normal to its surfaces. This feature results in lower pressure requirements to accomplish reduction in thickness to a given dimension.

It is therefore an object of this invention to provide a method of forging sheets more eificiently and more economically than by methods heretofore known.

Another object of this invention is to providea method of forging integrally stiffened panels of sheet metal of practical dimensions.

Itis still another object of this invention to provide a method for forging sheet material whereby larger pieces may be fabricated than possible with previously'known methods. and with available capacity of equipment.

A still further object of this invention is the provision of amethod for forging sheets whereby sheets of thinner final section may be produced, on any given press or hammer, than heretofore possible.

A still further object of this invention is the provision .of'a, method of die forging an article that may be later treated or processed to the desired shape by a mere bending operation, resulting in a product of greater size than the capacity of the press or hammer to'produce' directly.

Further objects and advantages will appear to those Thus, to effect a reduction in thickskilled in the art as the description proceeds in connection with the accompanying drawings, wherein:

Fig. l is a fragmentary sectional view of an integrally stiffened panel taken transverse to the ribs thereon and illustrating the approximate desired dimensions for the article;

Fig. 2 is a fragmentary sectional view of a portion of a preformed sheet of material from which the article of vFig. l is forged;

die structure from that shown in Figs. 3 to 6; and

Fig. 9 illustrates the product of the forging operation of Fig. 8.

Fig. 1 illustrates a portion of the desired product consisting of a relatively thin skin 2 of aluminum, aluminum alloy, or a suitable magnesium alloy or any other Iforgeable material for use in covering aircraft frames.

The skin 2 has integrally formed thereon spaced parallel upstanding ribs or flanges 4 joined integrally to one surfaceof the skin 2. In general, a satisfactory stiffened panel may have a skin portion of a thickness of the order of inch with the ribs also of about inch in thickness and projecting inwardly from skin 2 a distance of the order of 1 inch. The ribs may be from 1 /2 to 3 inches apart, although in some applications it is desirable that the ribs may be even closer or spaced apart a greater distance. Also the thickness dimensions and the height of the ribs may vary widely to meet particular requirements. In producing such shapes by forging, it is desirable that the ribs taper slightly toward their outer edge to provide for draft to facilitate removal from the dies.

The stock material from which the panel of Fig. l is produced consists of a relatively thick sheet of suitable metal and is preferably preformed to the sectional shape shown in Fig. 2. The preforming to the shape of Fig. 2 may be accomplished by forging between suitable dies 1 or may be performed by a rolling operation if desired.

Fig. 2 illustrates only a fragmentary portion of the preformed sheet illustrating only two rib positions. It is to be understood that such preformed sheet may extend laterally any desired distance within the limits of the fabricating machinery employed and will be provided with additional projections, like those shown. The preformed sheet 6 is formed with projections 8 and 1G spaced apart a distance less than ribs 4 of the final product and disposed on opposite sides thereof and having therein sufiicient metal to provide the metal necessary to form the ribs 4 in the final product. The portions of sheet 6 between the projections 38 or 19-10 provides the metal from which the skin portion 2 of the finished product is forged. In practicing the present invention any suitable forging press or hammer may be employed and is provided with a pair of dies 12 and 14 as shown in Figs. 3 to 6. The die 14 is provided with spaced parallel grooves 16 of a sectional shape corresponding to the shape and section of ribs 4. The portion of the die 14 between grooves 16 is formed as a rather broad deep channel 18 having oblique side walls 2i) and a curved bottom 22. The upper die 12 consists of a series of spaced parallel ribs 2 spaced apart a distance corresponding to the space between adjacent channels 18 in die 14. The channels between ribs 24 are relatively broad at their bases and are provided with a substantially flat bottom surface 26 joined to the oblique sides 28 of the ribs 24 by smoothly curved portions 30.

The preformed sheet 6 of Fig. 2 is so dimensioned that projections are readily received within the grooves 16 of die 14, as clearly shown in Fig. 3. The preformed sheet 6 is placed between the dies 12 and 14 as shown in Fig. 3 and the press set into operation whereupon the upper die .12 is moved down toward the die 14. Fig. 3 shows the relative positions of the parts as the crests of ribs 24 of the upper die 12 just reach the surface of the preformed sheet 6.

As the upper die 12 moves downwardly from the position shown in Fig. 3 the crests of ribs 24 depress the mid-portions of sheet 6 between the ribs 1%) and by a partial stretching and partial bending operation the preformed sheet is bent or corrugated to the shape shown in Fig. 4. As the die 12 moves downwardly still farther the condition shown in Fig. 5' is brought about wherein the bottom surfaces 26 of the channels between ribs .24 contact the upper portions of projections 8 and the material thereof starts flowing into grooves 16 to form the ribs. At the same time the material of the sheet 6 extends completely across the space between opposed oblique surfaces 20 and 28, of dies 14 and 12, respectively, and the pressure applied by the upper die 12 elfects forging of the material between those surfaces and causes that material to flow downwardly toward the bottom of channel 18. When this condition is reached practically all of the elongation of the sheet between grooves 16 is produced by a forging operation, which causes metal to fiow at a rate at least as great as that at which die 12 tends to stretch the sheet. 7

Referring specifically to Fig. 5 the broken line A-A indicates the dimension or thickness of metal between dies 12 and 14 measured in the direction of die travel. It is this dimension and its ratio to the distance, at right angles to this line, between points where there is an open space between the dies, into which the material may flow that determines the pressure necessary to cause flow of the metal, and as is perfectly obvious, the dimension A-A is considerably greater than the actual thickness of the sheet at that point, measured normal to its surfaces. Thus, actual forging of the metal may be accomplished by pressures considerably less than would be necessary if the sheet were forged between flat parallel surfaces extending normal to the direction of die travel.

As the upper die 12 continues to descend, the metal of the preformed sheet 6 is caused to flow to substantially fill all spaces between the dies. Fig. 6 illustrates the condition of the metal just prior to the time die 12 reaches the lowermost end of its stroke.

As die 12 moves downwardly from the position of Fig. 6 the metal adjacent the base of the ribs 4' is caused to flow sufliciently to completely fill grooves 16 and the web between the ribs will completely fill the space at the base of channels 18. When the parts have reached that position the forged sheet is-of the dimension and shape shown in Fig. 7, wherein the forging is shown as removed from the dies. 7

While not specifically described, it is contemplated that the above-described forging operation be performed while the material is maintained at an optimum temperature, as is well known in forging practice. The forging of aluminum or magnesium alloys is customarily performed in apparatus having provision for maintaining the dies and metal within a predetermined, relatively high temperature range for most efficient forging. It is also contcmplated that suitable lubricants will be employed between the material being forged and the dies, as is also well known.

Referring again to Feg. 7, the full line showing illustrates a fragment (in section) of the product, forged according to the sequence of Figs. 3 to 6, after it has been removed from the press. It will be noted that the ribs 4 are of the dimensions required and the same as those illustrated in Fig. 1. The skin or web-portion 2' is of the desired thickness, uniform throughout, and substantially that shown in Fig. l. The web or skin 2' is corrugated between the ribs 4 when it is removed from the press. The dies and sequence shown irrFigs; 3 to 6- suggest corrugating the web portion to a singl'ecorrugation but the invention isnot limited thereto; as be pointed out hereinafter. The forged corrugated sheet of Fig. 7 may be further treated to flatten thecorrugations between ribs 4 to define a substantially flat web, as shown indotted lines in Fig; 7. Upon flattening the skin portion 2 to a flat condition the ribs 4 thus be moved apart and will assume the required spacing since the actual length of web or skin material between the ribs; 4 is considerably greater than the actual spacing between grooves 16 of die 14, in which. the ribs were formed. Upon flattening the skin or web: between: ribs 4; the finished desired product is obtained The flattening operation may b pe f d by Stretching. the

forged sheet in a direction transverseto the. ribs, and suflicientlyt'o slightly exceed the elastic limit of the metal. The flattening may also beaccomplished hyother methods, such as. rolling. between suitable rollers;

8 illustrates: a modified form.whereinv the forging operation is carried out between: dies-likethosezofEig. 3- but shaped to form two corrugations betweerreach: pair of grooves 16. The lower die 14' is thus provided with two channels 18' and upper die12." is provided with a pair of ribs 24 between each of the grooves 16. The principles of operation are the same but it is to be noted that the material of the preformed: sheet fills the space between the upper and lower dies at the side walls: of the grooves and ribs sooner than. is the case. where a single corrugation is impressed in thez'nraterial and. actual forging of the web portion. between the. ribs commences at an, earlier stage in. the. travel of the" upper die 12.. Thus, lessstretching. of the material results; Many alloys. would be damaged. if. unduly'stretched during. the forging operation and such alloys can be satisfactorily forged according to the present invention by the method illustrated in Fig. 8. Furthermore, the dies of Fig. 8 may be more cheaply produced since less metal is removed, from the stock material, in their production.

Fig. 9 illustrates a fragment of the forged sheet resulting from the operation shown in Fig. 8. It is to be noted that the corrugations in this form are only approximately one-half the depth of the corrugation shown in Fig. 7 whereas the spacing between the ribs 4 will be the same as that shown in dotted lines in Fig. 7 after the forged sheet has been flattened, as described.

Referring now to Figs. 3, 7, 8 and 9, the portions of the preformed sheet 6 between the lines B (Figs 3 and 8) consists of the same volume of metal as is between the lines C of Figs 7 and 9. By so preforming sheet 6 that all the metal from which the ribs 4 are forged is located close to the mouths of the grooves 16 and between the high points of die 14, a minimum of flow is required to form the ribs and it is not necessary that the material thereof be forced a great distance laterally of the direction of die travel, as would be the case if the finished product were forged directly in the fiat and consequently much less pressure is required. When such shapes as shown in Fig. l are forged directly in the fiat, metal from the skin between ribs 4 must flow along the thin skin section and turn abruptly into the space defining the ribs 4. That abrupt change in direction coupled with high friction at the die surface causes the metal at 30 (Fig. 1) to follow the flow into the rib cavities and results in undesirable grooves or channels being formed opposite the bases of ribs 4. The formation of such defects is the suck-in previously referred to. As is clearly evident from Figs. 3 to 6, all of the material of ribs 4 flows into grooves 16 without appreciable change in direction and in response to pressure applied directly behind the flowing metal, in the direction of flow. The causes of suck-in, and consequently the defects themselves, are thus eliminated.

Clearly the dies may be so shaped that the finished product shown in dotted lines in Fig. 7 is a fiat sheet or may be shaped so that the finished? product is provided with curvature in one direction oreven' compound curvature. Throughout this: application-, andparticularly in the claims, the employment of the terms: substantially flat or generally flat is intended to. include other surface configurations such as cylindrical, elliptical or other forms.

In many applications of integrally stilfened panels it is desirable or necessary that certain areas of the finished panel bedevoid of ribs. For such applications the ribs 4 may be readily removed by a simple machine operanon.

While the present invention has been described in connection with the production of integrally stifiened panels for aircraft use, it. is to be understood: that the invention is not limited thereto but. may be employed in the production of other. structural. shapes and; may even be employed to thin. and elongate sheet materials. wherein no integral ribs are provided. In other words; the method of the present: invention; may also be employed in thinning and elongating fiat sheets of uniform. thickness;

While it is preferred that the corrugations between ribs 4 (as shown in Figs. 7 and9') be formed. to project into the space: between. the ribs", it is within. thescope of the present: invention to so forge; the: material that the corrugations project in. the opposite direction from the ribs.

While the present description is limited to a single mode of practicing the invention, it: is to be" understood that the invention is: not limited thereto but may embrace other or equivalent. steps' falling within thewscope of the appended claims. 1

1. In a method of forming integrally stiffened metal panels, the steps of: forging a sheet of stock material, between dies, to form integral ribs on one surface thereof, a predetermined distance apart, from material of said sheet substantially at the positions of said ribs, said forging step also reducing the thickness of and elongating said sheet in a direction extending between adjacent ribs and corrugating the same in a direction parallel to said ribs while holding said ribs said predetermined distance apart, the portions of said dies forming said corrugations comprising ribs and grooves having side faces oblique to the direction of die travel whereby the thickness of material forged therebetween, measured in the direction of die travel, is substantially greater than the actual thickness of said material measured normal to the surfaces thereof.

2. in a method of forming integrally stiffened metal panels, the steps of: forging a sheet of stock metal to form integral spaced ribs on one surface thereof at positions a fused distance apart from the material of said sheet substantially at the positions of said ribs, simultaneously thinning the stock material between said positions,- and elongating said material in a direction extending between said positions while forming at least one corrugation therein parallel to said ribs while holding said ribs said fixed distance apart, then flattening said corrugations to form a generally fiat continuous panel having integral stiflening ribs thereon spaced apart a distance greater than said fixed distance.

3. A method as defined in claim 2 wherein said corrugations are flattened by applying tension to said forged panel in a direction transverse to said ribs and sufiicient to slightly exceed the elastic limit of the metal.

4. In a method of forming integrally stiffened metal panels, the steps of: forging a sheet of stock metal having spaced enlargements thereon a fixed distance apart to form integral spaced ribs on one surface thereof from the material of said enlargements at positions said fixed distance apart, and working the material between said ribs by a combined stretching and compressing operation to thin said material, to increase its surface dimension between said rib positions, and to form at least one corrugation therein parallel to said ribs while holding said ribs said fixed distance apart. 7

5. .In a method of forming an integrally stifiEened metal panel, the steps of: preforming a sheet of stock material a to form spaced enlargements on one face thereof a predetermined distance apart, and while holding said enlargements said predetermined distance apart, forging said preformed sheet to form the material of said enlargements into spaced thin integral ribs on said one face of said sheet and simultaneously forging the material between said' ribs to reduce the thickness thereof and form at {east one corrugation therein between adjacent ribs and parallel to said ribs.

6. The method defined in claim 5 including the subsequent step of flattening said at least one corrugation between said adjacent ribs to form a substantially flat panel having integral stifiening libs on one surface thereof spaced apart a distance greater thanthe distance between said enlargements.

7. The method defined in claim 5 wherein said latter forging shapes the material between said ribs to form a single corrugation of a height substantially equal to the height of said ribs and extending 'therebetween.

8. The method defined in claim 5 wherein said latter forging shapes the material between said ribs to form two corrugations of a height equal to approximately one-half the height of said ribs and extending therebetween.

direction, said zones on one side of said sheet being bea tween those zones on the other side thereof, whereby to partially stretch said sheet in said one direction and cor: Y

one side overlap those on the other, and thereafter causing said pressure to squeeze metal frombetween the overlapping portions of said zones While continuing to deepen said corrugations, the said squeezing causing metal to flow from the overlapped portions of said zones at a rate substantially equal to the rate at which deepening said corrugations tend to stretch said sheet.

11. The method set forth in claim 10 wherein the overlapping portions of said zones extend oblique to said one direction along the sidesof said corrugations and wherein said pressure applied to all parts of said zones is in the same direction and substantially perpendicular to said one direction whereby the thickness of metal between overlapping portions of said zones, measured in the direction of the applied pressure, is greater than the actual thickness of the sheet. 7

References Cited in the file of this patent UNITED STATES PATENTS 

